Ovarian granulosa cell tumors occur in young SWR and (SWR X SJL)F1 but not in SJL mice. A genetic component is demonstrated in SWR X SJL recombinant inbred lines where one line has a 16% incidence, another line 1% incidence, and the remaining 12 lines are negative. Although hypothalamic-pituitary-ovarian endocrine functions are suppressed, these same mice are hyperthyroid through 7 months of age when rapid tumor growth is occurring. Multiple foci of granulosa cell tumors can be observed in follicles adjacent to normal appearing oocytes in pubertal ovaries. Evidence of viral activity associated with these tumors remains negative. Studies are in progress (1)\to determine the number of genes involved and their chromosomal location, (2)\to further characterize thyroid and other endocrine functions and (3)\to search for etiological factors and methods for suppression of granulosa cell tumors.